1. Field of Invention
The invention relates to a process for the manufacture of a soldered joint between a part made of pyrolytic graphite and a metal part. The invention further relates to a process for soldering pyrolytic graphite and metal parts and the soldered product produced thereby.
2. Description of Related Art
Because of its special properties, such as high thermal resistance, elevated heat conduction capacity and modest pulverizing rate, graphite is well suited as a thermal shield, among other things. At the same time, however, graphite possesses limited mechanical strength and modest ductility, so that in special applications it is only used as a material bonded with metals.
To this end, there are known processes in which polycrystalline graphite is soldered to suitable metallic materials, such as for example copper, inconel, molybdenum or a molybdenum alloy such as TZM which is a high temperature molybdenum alloy comprising 0.5% Ti, 0.08 Zr, 0.01-0.04% C and the balance molybdenum. Suitable solders include for example alloys of the elements Ag, Cu, Ni, Pd, Ti, Zr, Cr with portions of copper and titanium. Soldering takes place under vacuum, at temperatures between 850.degree. C. and 1900.degree. C., depending on the composition of the solder. Such bonded materials have proved valuable for example in the manufacture of structural components of the "first wall" or diverters and limiters in fusion reactions.
A graphite material featuring substantially superior properties compared to polycrystalline graphite, in terms of thermal resistance, heat conduction and a limited pulverization rate, is pyrolytic graphite. Pyrolytic graphite is produced by precipitation from a gaseous phase, whereby an orientation occurs of the atomic and/or crystallographic layers. The stratification of the pyrolytic graphite results in anisotropic properties, in contrast to polycrystalline graphite which possesses isotropic properties. Special after-treatment methods, such as annealing or simultaneous annealing and pressure application, result in the production of different qualities, such as annealed, and compression annealed, with slight differences in the individual characteristics.
Thus, compared to polycrystalline graphite, pyrolytic graphite exhibits substantially improved heat conduction, paralleling its stratification. At the same time, mechanical strength in this direction is very high, while thermal elongation is very modest. Conversely, heat conductivity and mechanical strength perpendicular to the stratification are limited, while thermal expansion is high.
Based on these special properties, it is difficult to solder pyrolytic graphite with metals which, as a rule, display isotropic properties.
Soldering processes which have proved valuable in soldering polycrystalline graphite with metals are not suited to the production of a satisfactory joint between pyrolytic graphite and metals. Above all, the use of such soldering processes leads to inadmissible cracks in the pyrolytic graphite, extending both parallel and perpendicular to its laminar strata.